Channel equalizer with acousto-optic variable attenuators

ABSTRACT

An optical communication assembly has an optical cross connect coupled to a first, a second, a third and a fourth set of optical fibers. A first demultiplexer is coupled to a first input fiber and the first set of optical fibers and a second demultiplexer is coupled to a second input fiber and the second set of optical fibers. A first multiplexer is coupled to a first output fiber and the third set of optical fibers. A second multiplexer is coupled to a second output fiber and the fourth set of optical fibers. A first set of attenuators is coupled to the third set of optical fibers and a second set of attenuators coupled to the fourth set of optical fibers.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims the benefit ofpriority from No. 60/206,767, filed May 23, 2000. This application isalso a CIP of Ser. No. 09/571,092, filed May 15, 2000, now U.S. Pat. No.6,253,002, which is a continuation of Ser. No. 09/425,099, filed Oct.22, 1999, now U.S. Pat. No. 6,233,379, which is a continuation-in-partof Ser. No. 09/022,413, filed Feb. 12, 1998 (now U.S. Pat. No.6,021,237), which claims priority to Korean Application No. 97-24796,filed Jun. 6, 1997, which applications are fully incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a variable optical attenuator (VOA),and more particularly to an all-fiber acousto-optic tunable intensityattenuator that is useful in optical telecommunications systems.

2. Description of Related Art

In modem telecommunication systems, many operations with digital signalsare performed on an optical layer. For example, digital signals areoptically amplified, multiplexed and demultiplexed. In long fibertransmission lines, the amplification function is performed by ErbiumDoped Fiber Amplifiers (EDFA's). The amplifier is able to compensate forpower loss related to signal absorption, but it is unable to correct thesignal distortion caused by linear dispersion, 4-wave mixing,polarization distortion and other propagation effects, and to get rid ofnoise accumulation along the transmission line. For these reasons, afterthe cascade of multiple amplifiers the optical signal has to beregenerated every few hundred kilometers. In practice, the regenerationis performed with electronic repeaters using optical-to-electronicconversion. However to decrease system cost and improve its reliabilityit is desirable to develop a system and a method of regeneration, orsignal refreshing, without optical to electronic conversion. An opticalrepeater that amplifies and reshapes an input pulse without convertingthe pulse into the electrical domain is disclosed, for example, in theU.S. Pat. No. 4,971,417, Radiation-Hardened Optical Repeater”. Therepeater comprises an optical gain device and an optical thresholdingmaterial producing the output signal when the intensity of the signalexceeds a threshold. The optical thresholding material such aspolydiacetylene thereby performs a pulse shaping function. The nonlinearparameters of polydiacetylene are still under investigation, and itsability to function in an optically thresholding device has to beconfirmed.

Another function vital to the telecommunication systems currentlyperformed electronically is signal switching. The switching function isnext to be performed on the optical level, especially in the WavelengthDivision Multiplexing (WDM) systems. There are two types of opticalswitches currently under consideration. First, there are wavelengthinsensitive fiber-to-fiber switches. These switches (mechanical, thermoand electro-optical etc.) are dedicated to redirect the traffic from oneoptical fiber to another, and will be primarily used for networkrestoration and reconfiguration. For these purposes, the switching timeof about 1 msec (typical for most of these switches) is adequate;however the existing switches do not satisfy the requirements for lowcost, reliability and low insertion loss. Second, there are wavelengthsensitive switches for WDM systems. In dense WDM systems having a smallchannel separation, the optical switching is seen as a wavelengthsensitive procedure. A small fraction of the traffic carried by specificwavelength should be dropped and added at the intermediate communicationnode, with the rest of the traffic redirected to different fiberswithout optical to electronic conversion. This functionality promisessignificant cost saving in the future networks. Existing wavelengthsensitive optical switches are usually bulky, power-consuming andintroduce significant loss related to fiber-to-chip mode conversion.Mechanical switches interrupt the traffic stream during the switchingtime. Acousto-optic tunable filters, made in bulk optic or integratedoptic forms, (AOTFs) where the WDM channels are split off by coherentinteraction of the acoustic and optical fields though fast, less thanabout 1 microsecond, are polarization and temperature dependent.Furthermore, the best AOTF consumes several watts of RF power, hasspectral resolution about 3 nm between the adjacent channels (which isnot adequate for current WDM requirements), and introduces over 5 dBloss because of fiber-to-chip mode conversions.

Another wavelength-sensitive optical switch may be implemented with atunable Fabry Perot filter (TFPF). When the filter is aligned to aspecific wavelength, it is transparent to the incoming optical power.Though the filter mirrors are almost 100% reflective no power isreflected back from the filter. With the wavelength changed or thefilter detuned (for example, by tilting the back mirror), the filterbecomes almost totally reflective. With the optical circulator in frontof the filter, the reflected power may be redirected from the incidentport. The most advanced TFPF with mirrors built into the fiber and PZTalignment actuators have only 0.8 dB loss. The disadvantage of thesefilters is a need for active feedback and a reference element forfrequency stability.

A VOA is in opto-mechanical device capable of producing a desiredreduction in the strength of a signal transmitted through a opticalfiber. Ideally, the VOA should produce a continuously variable signalattenuation while introducing a normal or suitable insertion loss andexhibiting a desired optical return loss. If the VOA causes excessivereflectance back toward the transmitter, its purpose will be defeated.

Due to the limitations of related art, there is a need for.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a VOA incombination with an electronic feedback loop.

This and other objects of the invention are achieved in an opticalcommunication assembly with an optical cross connect coupled to a first,a second, a third and a fourth set of optical fibers. A firstdemultiplexer is coupled to a first input fiber and the first set ofoptical fibers and a second demultiplexer is coupled to a second inputfiber and the second set of optical fibers. A first multiplexer iscoupled to a first output fiber and the third set of optical fibers. Asecond multiplexer is coupled to a second output fiber and the fourthset of optical fibers. A first set of attenuators is coupled to thethird set of optical fibers and a second set of attenuators coupled tothe fourth set of optical fibers.

In another embodiment of the present invention, an optical communicationassembly includes a demultiplexer coupled to an input fiber, amultiplexer coupled to an output fiber and a plurality of opticalfibers. Each optical fiber is coupled to one or both of thedemultiplexer and multiplexer. A plurality of attenuators are eachcoupled to an optical fiber. Each attenuator includes an attenuatoroptical fiber with a longitudinal axis, a core and a cladding in asurrounding relationship to the core. The attenuator optical fiber has aplurality of guided core modes. An acoustic wave propagation member,with a proximal end and a distal end, is included. The distal end iscoupled to the attenuator optical fiber. The acoustic wave propagationmember propagates an acoustic wave from the proximal to the distal endand launches an acoustic wave in the attenuator optical fiber. At leastone acoustic wave generator is coupled to the proximal end of theacoustic wave propagation member.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(a) is a schematic diagram of one embodiment of an AOTF of thepresent invention.

FIG. 1(b) is a cross-sectional view of the optical fiber of the FIG. 1AOTF.

FIG. 2 is a cross-sectional view of one embodiment of an acoustic wavepropagation member that can be used with the AOTF of FIG. 1.

FIG. 3(a) is a cross-sectional view illustrating one embodiment of aninterface created between an optical fiber and a channel formed in anacoustic wave propagation member of the FIG. 1 AOTF.

FIG. 3(b) is a cross-sectional view illustrating an embodiment of aninterface between an optical fiber and a channel formed in an acousticwave propagation member of the FIG. 1 AOTF where a bonding material isused.

FIG. 4 is a schematic diagram of one embodiment of an AOTF of thepresent invention with an acoustic damper.

FIG. 5 is a cross-sectional view of one embodiment of an index profileof an optical fiber, useful with the AOTF of FIG. 1, that has a doublingcladding.

FIG. 6 is a cross-sectional view of an optical fiber with sections thathave different diameters.

FIG. 7 is a cross-sectional view of an optical fiber with a taperedsection.

FIG. 8 is a perspective view of one embodiment of an AOTF of the presentinvention that includes a heatsink and two mounts.

FIG. 9 is a perspective view of one embodiment of an AOTF of the presentinvention with a filter housing.

FIG. 10 is a block diagram of an optical communication system with oneor more AOTF's of the present invention.

FIG. 11 is a schematic view showing the structure of an acousto-optictunable filter according to one embodiment of the present invention.

FIG. 12 is a graph showing the coupling and transmittance of the filterof FIG. 1.

FIG. 13 is a graph showing the transmittance of the filter of FIG. 11.

FIG. 14 is a graph showing the center wavelength of filter of FIG. 1 asa function of the frequency applied to the acoustic wave generator.

FIGS. 15(a)-(d) are graphs illustrating the transmissions of the filterof FIG. 11 when multiple frequencies are applied to the acoustic wavegenerator.

FIG. 16 includes graphs showing the transmittance characteristics of thefilter of FIG. 11 when varying an electric signal with a three frequencycomponent applied to the filter.

FIGS. 17(a)-(d) are graphs for comparing the mode convertingcharacteristic of the filter according to an embodiment of the presentinvention with that of a conventional wavelength filter.

FIG. 18(a) illustrate one embodiment of a transmission spectrum of theFIG. 11 filter.

FIG. 18(b) illustrates the measured and the calculated centerwavelengths of the notches as a function of acoustic frequency of anembodiment of the FIG. 11 filter.

FIG. 19 illustrates two examples of configurable spectral profiles withspectral tilt from the FIG. 11 filter.

FIG. 20(a) is a filter assembly that includes two filters of FIG. 11that are in series.

FIG. 20(b) is a schematic diagram of a dual-stage EDFA with a filter ofFIG. 20(a).

FIG. 21(a) is a graph of gain profiles of an EDFA with the filter ofFIG. 20(a).

FIG. 21(b) is a graph illustrating filter profiles that produced theflat gain profiles shown in FIG. 21(a).

FIG. 21(c) is a graph illustrating filter profiles of the FIG. 20(a)filter assembly.

FIGS. 22(a) and 22(b) are graphs illustrating the polarizationdependence of one embodiment of the filter of the present invention.

FIG. 23 illustrates one embodiment of the present invention from FIG. 4that has a reduction with a lower polarization dependent loss.

FIGS. 24(a) and 24(b) are graphs illustrating the polarization dependentloss profile of one embodiment of the invention, from FIG. 4, when thefilter is operated to produce 10-dB attenuation at 1550 nm.

FIG. 25 illustrates an embodiment of the invention with two of thefilters of FIG. 1.

FIG. 26 is a graph illustrating the effects of a backward acousticreflection at the damper of one embodiment of the present invention fromFIG. 4.

FIG. 27(a) is a graph illustrating, in one embodiment of FIG. 4, themodulation depth at 10-dB attenuation level at both first- andsecond-harmonics of the acoustic frequency.

FIG. 27(b) is a graph illustrating the modulation depth of first- andsecond-harmonics components from FIG. 27(a).

FIG. 28 is a schematic diagram of one embodiment of a VOA assembly ofthe present invention with a feedback loop.

FIG. 29 is a schematic diagram of another embodiment of a VOA of thepresent invention with a demultiplexer and a multiplexer.

FIG. 30 is a schematic diagram of an embodiment of a channel equalizerof the present invention using a single router.

FIG. 31 is a schematic diagram of another embodiment of a channelequalizer of the present invention using multiple routers.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of an AOTF (hereafter filter 10) ofthe present invention. An optical fiber 12 has a longitudinal axis, acore 14 and a cladding 16 in a surrounding relationship to core 14.Optical fiber 12 can be a birefringent or non-birefringent single modeoptical fiber and a multi-mode fiber. Optical fiber 12 can have multiplecladding modes and a single core mode guided along core 14, support coreto cladding modes and multiple cladding modes. Optical fiber 12 providesfundamental and cladding mode propagation along a selected length ofoptical fiber 12. Alternatively, optical fiber 12 is a birefringentsingle mode fiber that does not have multiple cladding modes and asingle core mode. In one embodiment, optical fiber 12 is tensioned.Sufficient tensioning can be applied in order to reduce losses in aflexure wave propagated in optical fiber 12.

The core of optical fiber 12 is substantially circular-symmetric. Thecircular symmetry ensures that the refractive index of the core mode isessentially insensitive to the state of optical polarization. Incontrast, in hi-birefringent single mode fibers the effective refractiveindex of the core mode is substantially different between two principalpolarization states. The effective refractive index difference betweenpolarization modes in high birefringence single mode fibers is generallygreater than 10⁻⁴. A highly elliptical core and stress-inducing membersin the cladding region are two main techniques to induce largebirefringence. In non-birefringent fibers, the effective indexdifference between polarization states is generally smaller than 10⁻⁵.

An acoustic wave propagation member 18 has a distal end 20 that iscoupled to optical fiber 12. Acoustic wave propagation member 18propagates an acoustic wave from a proximal end 22 to distal end 20 andlaunches a flexural wave in optical fiber 12. The flexural wave createsa periodic microbend structure in the optical fiber. The periodicmicrobend induces an antisymmetric refractive index change in the fiberand, thereby, couples light in the fiber from a core mode to claddingmodes. For efficient mode coupling, the period of the microbending, orthe acoustic wavelength, should match the beatlength between the coupledmodes. The beatlength is defined by the optical wavelength divided bythe effective refractive index difference between the two modes.

Acoustic wave propagation member 18 can be mechanically coupled to theoptical fiber and minimizes acoustic coupling losses in between theoptical fiber and the acoustic wave propagation member. In oneembodiment, acoustic wave propagation member 18 is coupled to opticalfiber 12 in a manner to create a lower order mode flexure wave inoptical fiber 12. In another embodiment, acoustic wave propagationmember 18 is coupled to the optical fiber to match a generation of modescarried by optical fiber 12.

Acoustic wave propagation member 18 can have a variety of differentgeometric configurations but is preferably elongated. In variousembodiments, acoustic wave propagation member 18 is tapered proximal end22 to distal end 20 and can be conical. Generally, acoustic wavepropagation member 18 has a longitudinal axis that is parallel to alongitudinal axis of optical fiber 12.

At least one acoustic wave generator 24 is coupled to proximal end 22 ofacoustic wave propagation member. Acoustic wave generator 24 can be ashear transducer.

Acoustic wave generator 24 produces multiple acoustic signals withindividual controllable strengths and frequencies. Each of the acousticsignals can provide a coupling between the core mode and a differentcladding mode. Acoustic wave generator 24 can produce multiple acousticsignals with individual controllable strengths and frequencies. Each ofthe acoustic signals provides a coupling between the core mode and adifferent cladding mode of optical fiber 12. A wavelength of an opticalsignal coupled to cladding 16 from core 14 is changed by varying thefrequency of a signal applied the acoustic wave generator 24.

Acoustic wave generator 24 can be made at least partially of apiezoelectric material whose physical size is changed in response to anapplied electric voltage. Suitable piezoelectric materials include butare not limited to quartz, lithium niobate and PZT, a composite of lead,zinconate and titanate. Other suitable materials include but are notlimited to zinc monoxide. Acoustic wave generator 24 can have amechanical resonance at a frequency in the range of 1-20 MHz and becoupled to an RF signal generator.

Referring now to FIG. 2, one embodiment of acoustic wave propagationmember 18 has an interior with an optical fiber receiving channel 26.Channel 26 can be a capillary channel with an outer diameter slightlygreater than the outer diameter of the fiber used and typically in therange of 80˜150 microns. The length of the capillary channel ispreferably in the range of 5˜15 mm. The interior of acoustic wavepropagation member 18 can be solid. Additionally, acoustic wavepropagation member 18 can be a unitary structure.

Optical fiber 12 is coupled to acoustic wave propagation member 18. Asillustrated in FIG. 3(a), the dimensions of channel 26 and an outerdiameter of optical fiber 12 are sufficiently matched to place the twoin a contacting relationship at their interface. In this embodiment, therelative sizes of optical fiber 12 and channel 26 need only besubstantially the same at the interface. Further, in this embodiment,the difference in the diameter of optical fiber 12 and channel 26 are inthe range of 1˜10 microns

In another embodiment, illustrated in FIG. 3(b), a coupling member 28 ispositioned between optical fiber 12 and channel 26 at the interface.Suitable coupling members 28 including but are not limited to bondingmaterials, epoxy, glass solder, metal solder and the like.

The interface between channel 26 and optical fiber 12 is mechanicallyrigid for efficient transduction of the acoustic wave from the acousticwave propagation member 18 to the optical fiber 12.

Preferably, the interface between optical fiber 12 and channel 26 issufficiently rigid to minimize back reflections of acoustic waves fromoptical fiber 12 to acoustic wave propagation member 18.

In the embodiments of FIGS. 3(a) and 3(b), acoustic wave propagationmember 18 is a horn that delivers the vibration motion of acoustic wavegenerator 24 to optical fiber 12. The conical shape of acoustic wavepropagation member 18, as well as its focusing effect, providesmagnification of the acoustic amplitude at distal end 20, which is asharp tip. Acoustic wave propagation member 18 can be made from a glasscapillary, such as fused silica, a cylindrical rod with a central hole,and the like.

In one embodiment, a glass capillary is machined to form a cone and aflat bottom of the cone was bonded to a PZT acoustic wave generator 24.Optical fiber 12 was bonded to channel 26. Preferably, distal end 20 ofacoustic wave generator 18 is as sharp as possible to minimizereflection of acoustic waves and to maximize acoustic transmission.Additionally, the exterior surface of acoustic wave generator 18 issmooth. In another embodiment, acoustic wave generator 18 is a horn witha diameter that decreases exponentially from proximal end 22 to distalend 20.

As illustrated in FIG. 4, filter 10 can also include an acoustic damper30 that is coupled to optical fiber 12. Acoustic damper 30 includes ajacket 32 that is positioned in a surrounding relationship to opticalfiber 12. Acoustic damper 30 absorbs incoming acoustic waves andminimizes reflections of the acoustic wave. The reflected acoustic wavecauses an intensity modulation of the optical signal passing through thefilter by generating frequency sidebands in the optical signal. Theintensity modulation is a problem in most applications. A proximal end34 of the acoustic damper 30 can be tapered. Acoustic damper 30 can bemade of a variety of materials. In one embodiment, acoustic damper 30 ismade of a soft material that has a low acoustic impedance so thatminimizes the reflection of the acoustic wave. Jacket 32 itself is asatisfactory damper and in another embodiment jacket 32 takes the placeof acoustic damper 30. Optionally, jacket 32 is removed from thatportion of optical fiber 12 in a interactive region 36 and that portionof optical fiber 12 that is bonded to acoustic wave generator 24.

The interactive region is where an optical signal is coupled to cladding16 from core 14. This coupling is changed by varying the frequency of asignal applied to acoustic wave generator 24. In one embodiment,interactive region 36 extends from distal end 20 to at least a proximalportion within acoustic damper 30. In another embodiment, interactiveregion 36 extends from distal end 20 and terminates at a proximal end ofacoustic damper 30. In one embodiment, the length of optical fiber 12 ininteractive region is less than 1 meter, and preferably less than 20 cm.The nonuniformity of the fiber reduces the coupling efficiency and alsocauses large spectral sidebands in the transmission spectrum of thefilter. Another problem of the long length is due to the modeinstability. Both the polarization states of the core and cladding modesand the orientation of the symmetry axis of an antisymmetric claddingmode are not preserved as the light propagates over a long lengthgreater than 1 m. This modal instability also reduces the couplingefficiency and causes large spectral sidebands. Preferably, the outerdiameter of optical fiber 12, with jacket 32, is in the range of 60-150microns.

The profile of the refractive index of the cross section of opticalfiber 12 influences its filtering characteristics. One embodiment ofoptical fiber 12, illustrated in FIG. 5, has a first and second cladding16′ and 16″ with core 14 that has the highest refractive index at thecenter. First cladding 16′ has an intermediate index and second cladding16″ has the lowest index. Most of the optical energy of severallowest-order cladding modes is confined both only in core 14 and firstcladding 16′. The optical energy falls exponentially from the boundarybetween first and second claddings 16′ and 16″, respectively.

Optical fields are negligible at the interface between second cladding16″ and the surrounding air, the birefringence in the cladding modes,due to polarization-induced charges, is much smaller than inconventional step-index fibers where second cladding 16″ does not exist.The outer diameter of first cladding 16′ is preferably smaller than thatof second cladding 16″, and can be smaller by at least 5 microns. In onespecific embodiment, core 14 is 8.5 microns, first cladding 16′ has anouter diameter of 100 microns and second cladding 16″ has an outerdiameter of 125 microns. Preferably, the index difference between core14 and first cladding 16′ is about 0.45%, and the index differencebetween first and second claddings 16′ and 16″ is about 0.45%.

In another embodiment, the outer diameter of first cladding 16′ issufficiently small so that only one or a few cladding modes can beconfined in first cladding 16′. One specific example of such an opticalfiber 12 has a core 14 diameter of 4.5 microns, first cladding 16′ of 10microns and second cladding 16″ of 80 microns, with the index differencebetween steps of about 0.45% each.

The optical and acoustic properties of optical fiber 12 can be changedby a variety of different methods including but not limited to, (i)fiber tapering, (ii) ultraviolet light exposure, (iii) thermalstress-annealing and (iv) fiber etching.

One method of tapering optical fiber 12 is achieved by heating andpulling it. A illustration of tapered optical fiber 12 is illustrated inFIG. 6. As shown, a uniform section 38 of narrower diameter is createdand can be prepared by a variety of methods including but not limited touse of a traveling torch. Propagation constants of optical modes can begreatly changed by the diameter change of optical fiber 12. The pullingprocess changes the diameter of core 14 and cladding 16 and also changesthe relative core 14 size due to dopant diffusion. Additionally, theinternal stress distribution is modified by stress annealing. Taperingoptical fiber 12 also changes the acoustic velocity.

When certain doping materials of optical fiber 12 are exposed toultraviolet light their refractive indices are changed. In oneembodiment, Ge is used as a doping material in core 14 to increase theindex higher than a pure SiO₂ cladding 16. When a Ge-doped optical fiber12 is exposed to ultraviolet light the index of core 14 can be changedas much as 0.1%. This process also modifies the internal stress fieldand in turn modifies the refractive index profile depending on theoptical polarization state. As a result, the birefringence is changedand the amount of changes depends on optical modes. This results inchanges of not only the filtered wavelength at a given acousticfrequency or vice versa but also the polarization dependence of thefilter. Therefore, the UV exposure can be an effective way of trimmingthe operating acoustic frequency for a given filtering wavelength aswell as the polarization dependence that should preferably be as smallas possible in most applications.

Optical fiber 12 can be heated to a temperature of 800 to 1,300° C. orhigher to change the internal stresses inside optical fiber 12. Thisresults in modification of the refractive index profile. The heattreatment is another way of controlling the operating acoustic frequencyfor a given filtering wavelength as well as the polarization dependence.

The propagation velocity of the acoustic wave can be changed bychemically etching cladding 16 of optical fiber 12. In this case, thesize of core 14 remains constant unless cladding is completely etched.Therefore, the optical property of core mode largely remains the same,however, that of a cladding mode is altered by a different claddingdiameter. Appropriate etchants include but are not limited to hydrofluoride (HF) acid and BOE.

The phase matching of optical fiber 12 can be chirped. As illustrated inFIG. 6, a section 40 of optical fiber can have an outer diameter thatchanges along its longitudinal length. With section 40, both the phasematching condition and the coupling strength are varied along its z-axis42 and the phase matching conditions for different wavelengths satisfiedat different positions along the axis. The coupling then can take placeover a wide wavelength range. By controlling the outer diameter as afunction of its longitudinal axis 42, one can design varioustransmission spectrum of the filter. For example, uniform attenuationover a broad wavelength range is possible by an appropriate diametercontrol.

Chirping can also be achieved when the refractive index of core 14 isgradually changed along z-axis 42. In one embodiment, the refractiveindex of core 14 is changed by exposing core 14 to ultraviolet lightwith an exposure time or intensity as a function of position along thelongitudinal axis. As a result, the phase matching condition is variedalong z-axis 42. Therefore, various shapes of transmission spectrum ofthe filter can be obtained by controlling the variation of therefractive index as a function of the longitudinal axis 42.

As illustrated in FIG. 8 a heatsink 44 can be included to cool acousticwave generator. In one embodiment, heatsink 44 has a proximal face 46and a distal face 48 that is coupled to the acoustic wave generator 24.Preferably, acoustic wave generator 24 is bonded to distal face 48 byusing a low-temperature-melting metal-alloy solder including but notlimited to a combination of 95% zinc and 5% tin and indium-based soldermaterials. Other bonding material includes heat curable silver epoxy.The bonding material should preferably provide good heat and electricalconduction. Heatsink 44 provides a mount for the acoustic wave generator24. Heatsink can be made of a variety of materials including but notlimited to aluminum, but preferably is made of a material with a highheat conductivity and a low acoustic impedance.

Acoustic reflections at proximal face can be advantageous if controlled.By introducing some amount of reflection, and choosing a right thicknessof heatsink 44, the RF response spectrum of acoustic wave generator 24can be modified so the overall launching efficiency of the acoustic wavein optical fiber can be less dependent on the RF frequency.

In this case, the reflectivity and size of heatsink 44 is selected toprovide a launching efficiency of the flexural wave into optical fiber12 almost independent of an RF frequency applied to acoustic wavegenerator 24. The thickness of heatsink 44 is selected to provide atravel time of an acoustic wave from distal face 48 to proximal face 46,and from proximal face 46 to distal face 48 that substantially matches atravel time of the acoustic wave traveling through acoustic wavepropagation member 24 from its proximal end to its distal end, and fromits distal end to its proximal end. The heat sink material or thematerial for the attachment to the proximal face 46 is selected toprovide the amount of back reflection from the heat sink thatsubstantially matches the amount of back reflection from the acousticwave propagation member. In various embodiments, the proximal and distalfaces, 46, 48 of heatsink 44 have either rectangular or circular shapeswith the following dimensions: 10×10 mm² for the rectangular shape anddiameter of 10 mm for the cylindrical shaped heat sink.

However, acoustic back reflections due to proximal face 46 arepreferably avoided. Acoustic reflections from the heat sink back to theacoustic wave generator are reduced by angling proximal face 46 at anangle greater than 45 degree or by roughing the face. The acoustic wavecoming from the acoustic generator toward the angled proximal face 46 isreflected away from the acoustic generator, reducing the acoustic backreflection to the acoustic wave generator. The roughed face also reducesthe acoustic reflection by scattering the acoustic wave to randomdirections. Preferably, the side faces of the heat sink are alsoroughened or grooved to scatter the acoustic wave and thereby to avoidthe acoustic back reflection. Another method to reduce the backreflection is to attach an acoustic damping material at the proximalface 46. Suitable materials that reduce back reflections include softpolymers, silicone, and the like that can be applied to proximal face46.

Referring again to FIG. 8, an acoustic damper mount 50 supports acousticdamper 30. Acoustic damper mount 50 can be made of a variety ofmaterials including but not limited to silica, invar, and the like. Afilter mount 52 supports heatsink 44 and acoustic damper mount 50. Inone embodiment, filter mount is a plate-like structure. Preferably,filter mount 52 and optical fiber 12 have substantially the same thermalexpansion coefficients. Filter mount 52 and fiber 12 can be made of thesame materials.

Filter mount 52 and optical fiber 12 can have different thermalexpansion coefficients and be made of different materials. In oneembodiment, filter mount 52 has a lower thermal expansion coefficientthan optical fiber 12. Optical fiber 12 is tensioned when mounted andbonded to the filter mount 52. The initial strain on optical fiber 12 isreleased when the temperate increases because the length of filter mount52 is increased less than optical fiber 12. On the other hand, when thetemperature decreases optical fiber 12 is stretched further. When theamount of strain change according to temperature change is appropriatelychosen by selecting proper material for mount the 52, the filteringwavelength of filter 10 can be made almost independent of temperature.Without such mounting arrangement, the center wavelength of the filterincreases with temperature. Additionally, interactive region 36 of issufficiently tensioned to compensate for changes in temperature of theinteractive region 36 and filter mount 52.

In another embodiment, illustrated in FIG. 9, a filter housing 54encloses interactive region 36. Filter housing 54 can be made of avariety of materials, including but not limited to silica, invar and thelike. Filter housing 54 eliminates the need for a separate filter mount52. Filter housing 54 extends from distal face 48 of heatsink 44 toacoustic damper 30 or to a jacketed portion 32 of optical fiber 12.Acoustic wave propagation member 18, acoustic wave generator 24 and theacoustic damper 30 can be totally or at least partially positioned in aninterior of filter housing 54.

In one embodiment, filter housing 54 and optical fiber 12 are made ofmaterials with substantially similar thermal expansion coefficients. Asuitable material is silica. Other materials are also suitable andinclude invar. Filter housing 54 and optical fiber 12 can have differentthermal expansion coefficients and be made of different materials. Inone embodiment, filter housing 54 has a lower thermal expansioncoefficient than optical fiber 12.

In one embodiment, interactive region 36 is sufficiently tensionedsufficiently to compensate for changes in temperature of interactiveregion 36 and filter housing 54.

As illustrated in FIG. 10, filter 10 can be a component or subassemblyof an optical communication system 56 that includes a transmitter 58 anda receiver 60. Transmission 58 can include a power amplifier with filter10 and receiver 60 can also include a pre amplifier that includes filter10. Additionally, optical communication system 56 may also have one ormore line amplifiers that include filters 10.

Referring now to FIG. 11, if an electric signal 57 with constantfrequency “f” is applied to acoustic wave generator 24, a flexuralacoustic wave having the same frequency “f” is generated. The flexuralacoustic wave is transferred to optical fiber 12 and propagates alongoptical fiber 12, finally absorbed in acoustic damper 30. The flexuralacoustic wave propagating along optical fiber 12 produces periodicmicrobending along the fiber, resulting in the periodic change ofeffective refractive index which the optical wave propagating alongoptical fiber 12 experiences. The signal light propagating along opticalfiber 12 in a core mode can be converted to a cladding mode by thechange of effective refractive index in optical fiber 12.

When signal light is introduced into filter 10 part of the signal lightis converted to a cladding mode due to the effect of the acoustic waveand the remainder of the signal light propagates as a core mode whilethe signal light propagates along interactive region 36. The signallight converted to a cladding mode cannot propagate any longer inoptical fiber 12 with jacket 32 because the light is partly absorbed inoptical fiber 12 or partly leaks from optical fiber 12. A variety ofmode selecting means, including a mode conversion means between coremodes and cladding modes, can be incorporated in filter 10. For example,the long-period grating described in the article “Long-periodfiber-grating based gain equalizers” by A. M. Vengsarkar et al. inOptics Letters, Vol. 21, No. 5, p. 336, 1996 can be used as the modeselecting means. As another example, a mode coupler, which converts oneor more cladding modes of one fiber to core modes of the same fiber oranother fiber, can also be used.

A flexural acoustic wave generated by acoustic wave propagation member18 propagates along interactive region 36. The acoustic wave createsantisymmetric microbends that travel along interactive region 36,introducing a periodic refractive-index perturbation along optical fiber12. The perturbation produces coupling of an input symmetric fundamentalmode to an antisymmetric cladding mode when the phase-matching conditionis satisfied in that the acoustic wavelength is the same as the beatlength between the two modes. The coupled light in the cladding mode isattenuated in jacket 32. For a given acoustic frequency, the couplingbetween the fundamental mode and one of the cladding modes takes placefor a particular optical wavelength, because the beat length hasconsiderable wavelength dispersion. Therefore, filter 10 can be operatedas an optical notch filter. A center wavelength and the rejectionefficiency are tunable by adjustment of the frequency and the voltage ofRF signal applied to acoustic wave propagation member 18, respectively.

The coupling amount converted to a cladding mode is dependent on thewavelength of the input signal light. FIG. 12(a) shows the couplingamounts as functions of wavelength when flexural acoustic waves at thesame frequency but with different amplitudes are induced in opticalfiber 12. As shown in FIG. 12(a), the coupling amounts are symmetricalwith same specific wavelength line (λ_(c)), i.e.,. center wavelengthline, however they show different results 62 and 64 due to the amplitudedifference of the flexural acoustic waves. Therefore, the transmittanceof the output light which has passed through filter 12 is differentdepending on the wavelength of the input light. Filter 12 can act as anotch filter which filters out input light with specific wavelength asshown in FIG. 12(b).

FIG. 12(b) is a graph showing the transmittances as a function ofwavelength when flexural acoustic waves with different amplitudes areinduced in optical fiber 12. The respective transmittances have samecenter wavelength as does the coupling amount, but differenttransmittance characteristic 64 and 66 depending on the amplitudedifference of the flexural acoustic waves can be shown.

The center wavelength λ_(c), of filter 10 satisfies the followingequation.

β_(co)(λ)−β_(cl)(λ)=2π/λ_(a)

In the above equation, β_(co)(λ) and β_(cl)(λ) are propagation constantsof core mode and cladding mode in optical fiber 12 which arerespectively dependent on the wavelength, and λ_(a) represents thewavelength of the flexural acoustic waves.

Accordingly, if the frequency of the electric signal applied to acousticwave generator 24 varies, the wavelength of the acoustic wave generatedin optical fiber 12 also varies, which results in the center wavelengthchange of filter 10. In addition, since the transmission is dependent onthe amplitude of the flexural acoustic wave, the transmission of signallight can be adjusted by varying the amplitude of the electric signalwhich is applied to acoustic wave generator 24.

FIG. 13 is a graph showing the transmittance of filter 10 in oneembodiment when different electric signal frequencies are applied. Asshown in FIG. 13, each center wavelength (i.e., wavelength showingmaximum attenuation) of filter 10 for different electric signals was1530 nm, 1550 nm and 1570 nm. Therefore the center wavelength of filter10, according to the embodiment, is changed by varying the frequency ofthe electric signal which is applied to acoustic wave generator 24.

As described above, since there are a plurality of cladding modes ininteractive region 36 the core mode can be coupled to several claddingmodes. FIG. 14 is a graph showing the center wavelength of filter 10according to the embodiment of the invention as a function of thefrequency applied to the flexural acoustic wave generator. In FIG. 14,straight lines 71, 72 and 73 represent the center wavelength of filter10 resulting from the coupling of a core mode with three differentcladding modes.

Referring to FIG. 14, there are three applied frequencies for any oneoptical wavelength in this case. Therefore the input signal light isconverted to a plurality of cladding modes by applying multi-frequencyelectric signal to acoustic wave generator 24. Moreover, it meanstransmission characteristics of filter 10 can be electrically controlledby adjusting the amplitude and each frequency component of the electricsignal.

As shown in FIG. 15(a), the respective transmission features 74, 75 and76 of filter 10 can be provided by applied electric signals withdifferent frequencies f1, f2 and f3. In this example, assuming that f1couples the core mode of input signal light to a cladding mode (claddingmode A), f2 couples the core mode to other cladding mode (cladding modeB) and f3 couples the core mode to another cladding mode different fromA or B (cladding mode C, the transmission feature is shown in FIG. 15(b)as a curve 77 when electric signal with three frequency components f1,f2 and S is applied to acoustic wave generator 24.

As shown in FIG. 5(c), if filter 10 has transmission feature curves 78,79 and 80 corresponding to respective frequencies f1′, f2′ and f3′ andelectric signal having three frequency components f1′, f2′ and f3′ isapplied to the flexural acoustic wave generator, the transmissionfeature of filter 10 is shown as a curve 81 of FIG. 5(d).

FIG. 16 includes graphs showing the transmittance of filter 10 accordingto an embodiment of the present invention, when varying electric signalhaving three frequency components is applied to filter 10. When varyingelectric signal having a plurality of frequency components is applied toacoustic wave generator 24 various shapes of transmittance curves 82, 83and 84 can be obtained.

Since conventional tunable wavelength filters utilize the coupling ofonly two modes, the difference between a plurality of appliedfrequencies naturally becomes small to obtain wide wavelength bandfiltering feature by applying a plurality of frequencies. In this case,as described under the article “Interchannel Interference inmultiwavelength operation of integrated acousto-optical filters andswitches” by F. Tian and H. Herman in Journal of Light wave technology1995, Vol. 13, n 6, pp. 1146-1154, when signal light input to a filteris simultaneously converted into same (polarization) mode by variousapplied frequency components, the output signal light may undesirably bemodulated with frequency corresponding to the difference between theapplied frequency components. However, with filter 10 the above problemcan be circumvented, because the respective frequency components convertthe mode of input light into different cladding modes in filter 10.

In one embodiment, the filtering feature shown in FIG. 17(a) wasobtained by applying adjacent frequencies 2.239 MHz and 2.220 MHz toreproduce the result of a conventional method. The applied twofrequencies were such that convert the mode of input light into the samecladding mode. Under the condition, narrow wavelength-band signal lightwith a center wavelength of 1547 nm was input to filter 10 to measureoutput light. Referring to the measurement result shown in FIG. 17(b),there is an undesirable modulated signal with frequency corresponding tothe difference of the two applied frequencies.

In another embodiment, when adjacent frequencies 2.239 MHz and 2.220 MHzwere applied to acoustic wave generator 24, according to the embodimentof the invention, the two frequency components convert the mode of inputlight into mutually different cladding modes. FIG. 17(c) shows themeasurement result when the same signal light as the above experimentwas input to filter 10 and output light was measured.

However, an undesirable modulated signal, which appeared in aconventional filter, practically disappeared as shown in FIG. 17(d).

In optical communications or optical fiber sensor systems, wavelengthfilters are required that has a wide tuning range and are capable ofelectrically controlling its filtering feature.

FIG. 18(a) illustrates one embodiment of a transmission spectrum offilter 10 with a 15.5-cm-long interaction length for a broadbandunpolarized input light from a LED. A conventional communication fiberwas used with a nominal core diameter of 8.5 μm, a cladding outerdiameter of 125 μm and a normalized index difference of 0.37%. Thefrequency of the applied RF signal was 2.33 MHz, and the correspondingacoustic wavelength was estimated to be ˜650 μm. The three notches shownin FIG. 8(a) are from the coupling to three different cladding modeswith the same beat length at the corresponding wavelengths. The coupledcladding modes were the LP₁₁ ^((cl)), the LP₁₂ ^((cl)), and the LP₁₃^((cl)) modes, which was confirmed from far-field radiation patterns.The center of each coupling wavelength was tunable over >100 nm bytuning the acoustic frequency.

FIG. 18(b) shows the measured and the calculated center wavelengths ofthe notches as a function of acoustic frequency. The fiber parametersused in the calculation for best fit with the experimental results are acore diameter of 8.82 μm, a cladding outer diameter of 125 μm, and anormalized index difference of 0.324%, in reasonable agreement with theexperimental fiber parameters.

Referring again to FIG. 8(a), coupling light of a given wavelength fromthe fundamental mode to different cladding modes requires acousticfrequencies that are separated from each other by a few hundredkilohertz. This separation is large enough to provide a widewavelength-tuning range of almost 50 nm for each coupling mode pairwithout significant overlap with each other, thereby practicallyeliminating the coherent cross talk that is present in conventionalcounterparts. The tuning range is sufficient to cover the bandwidth oftypical EDFA's. In one embodiment, filter 10 provides for a combinationof independent tunable notch filters built into one device, and thenumber of involved cladding modes corresponds to the number of filters.The multifrequency acoustic signals can be generated by a singletransducer, and the spectral profile of filter 10 is determined by thefrequencies and amplitudes of the multiple acoustic signals.

FIG. 19 shows two examples of the configurable spectral profiles withspectral tilt, which can be used to recover the gain flatness in an EDFAwith a gain tilt caused by signal saturation. In one embodiment, threecladding modes [LP₁₁ ^((cl)), the LP₁₂ ^((cl)), and the LP₁₃ ^((cl))]were used and three RF signals were simultaneously applied withdifferent voltages and frequencies adjusted for the particular profile.The 3-dB bandwidth of the individual notch was ˜6 nm with a 10-cm-longinteraction length.

A complex filter profile is required to flatten an uneven EDFA gain,which exhibits large peaks with different widths around 1530 and 1560nm. The combination of three Gaussian shaped passive filters can producea flat gain over a 30-nm wavelength range.

As illustrated in FIG. 20(a), a filter assembly of the present inventioncan include first and second filters 10′ and 10″ in series. Each filter10′ and 10″ is driven by three radio frequency (RF) signals at differentfrequencies and amplitudes that produce acousto-optic mode conversionfrom the fundamental mode to different cladding modes. This approacheliminates the detrimental coherent crosstalk present in LiNbO₃-basedAOTF's. The 3-dB bandwidths of the first filter 10′ were 3.3, 4.1, and4.9 nm for the couplings to the cladding modes LP₁₂ ^((cl)), the LP₁₃^((cl)), and LP₁₄ ^((cl)), respectively. For second filter 10″, theywere 8, 8.6, and 14.5 nm for the couplings to the cladding modes, LP₁₁^((cl)), the LP₁₂ ^((cl)), and the LP₁₃ ^((cl)) respectively.

The minimum separations of notches produced by single RF drivingfrequency were ˜50 nm for first filter 10′ and ˜150 nm for second filter10″, respectively, so that only one notch for each driving frequencyfalls into the gain-flattening range (35 nm). The large differencebetween filters 10′ and 10″ was due to the difference in optical fiber12 outer diameters. The polarization splitting in the center wavelengthof the notches as ˜0.2 nm for first filter 10′ and ˜1.5 nm for secondfilter 10″. The relatively large polarization dependence in secondfilter 10″ is due mainly due to the unwanted core elliptically andresidual thermal stress in optical fiber 12, that can be reduced to anegligible level by using a proper optical fiber. First and secondfilters 10′ and 10″ were used for the control of the EDFA gain shapearound the wavelengths of 1530 and 1555 nm, respectively. The backgroundloss of the gain flattening AOTF was less than 0.5 dB, which was mainlydue to splicing of different single-mode fibers used in the two AOTF's1010. Adjusting the frequencies and voltages of the applied RF signalsprovided control of the positions and depths of the notches with greatflexibility. The RF's were in the range between 1 and 3 MHz.

FIG. 20(b) shows a schematic of a dual-stage EDFA employing gainflattening filter 10 along with a test setup. A 10-m-long EDF pumped bya 980-nm laser diode and a 24-m-long EDF pumped by a 1480-nm laser diodewere used as the first and the second stage amplifiers, respectively.The peak absorption coefficients of both EDF's were ˜2.5 dB/m at 1530nm. Filter 10 was inserted between the two stages along with anisolator. Total insertion loss of filter 10 and the isolator was lessthan 0.9 dB. Six synthesizers and two RF power amplifiers were used todrive filter 10.

Gain profiles of the EDFA were measured using a saturating signal at thewavelength of 1547.4 nm and a broad-band light-emitting diode (LED)probing signal. The saturating signal from a distributed feedback (DFB)laser diode was launched into the EDFA after passing through aFabry-Perot filter (optical bandwidth: 3 GHz, extinction ratio: 27 dB)to suppress the sidelobes of the laser diode. The total power of theprobe signal in 1520-1570-nm range was 27 dBm, which is much smallerthan that of the input saturating signal ranging from 13 to 7 dBm.

FIG. 21(a) shows gain profiles before and after the gain flattening fortwo different saturating signal powers of 13 and 7 dBm when thesecond-stage pump power was 42 mW. The gain excursions before flatteningwere larger than 5 dB. By adjusting the filter profile, flat gainprofiles within 0.7 dB were obtained over 35 nm for both cases. The flatgain region is shifted slightly toward the shorter wavelength for highergain level, which is due to the intrinsic gain characteristics of theEDF. FIG. 21(b) shows filter profiles that produced the flat gainprofiles shown in FIG. 21(a), where Profile 1 and Profile 2 are for thecases of saturating tones of 13 and 7 dBm, respectively. For themeasurements, EDFI was used as an ASE source, while the second pumpdiode (1480 nm) was turned off. The ASE signal leaked out of the secondWDM coupler was monitored and the signals obtained when the filter wason and off were compared to yield the filter response. The attenuationcoefficients for Profile 1 and Profile 2 at the saturating signalwavelength were 5.0 and 4.9 dB, respectively, and the averageattenuation over the 35-nm range (1528-1563 nm) was 5 dB in both cases.The total RF electrical power consumption of the filter was less than500 mW. Profile 2 could be obtained from Profile 1 by adjusting mainlythe depths of notches, although fine adjustments of center wavelengthsof notches within 0.5-nm range slightly improved the gain flatness. FIG.21(c) shows the filter profiles of first filter 10′ and second filter10″ used to form Profile 1, and also the locations of center wavelengthsof six notches. By adjusting first and second filters 10′ and 10″spectral profiles electronically gain flatness of <0.7 dB over 35-nmwavelength range were obtained at various levels of gain as well asinput signal and pump power.

One important characteristic of filter 10 is polarization dependence.The shape of filter 10 can be dependent on the polarization state ofinput light. The polarization dependence originates from fiberbirefringence. Fiber birefringence causes the effective propagationconstant of a mode to be different between two eigen polarizationstates. Since the magnitude of birefringence is different from mode tomode, the fiber birefringence causes the beat length between two coupledmodes to be different between the eigen polarization states, and,therefore, results in splitting of center wavelength of filter 10 for agiven acoustic frequency.

FIG. 22(a) illustrates the polarization dependence. Curve 90 representsthe filter profile for light in one eigen polarization state, and filterprofile 92 is when the input is in the other eigen state. The centerwavelengths are split because of the birefringence. Moreover, since thefield overlap between two coupled modes is also polarization dependentdue to the birefringence, the attenuation depth can be different betweenfilter profiles 90 and 92.

A critical feature due to the polarization dependence is thepolarization dependent loss (PDL) which is defined as the difference ofthe magnitude of attenuation between two eigen polarization states.Since polarization dependent loss is an absolute value, it increaseswith the attenuation depth. FIG. 22(b) shows polarization dependent lossprofile 93 associated with filter profiles 90 and 92. In WDMcommunication system applications, the polarization dependent lossshould be minimized. Most applications require the polarizationdependent loss to be less than 0.1 dB. However, acousto-optic tunablefilter 10 has exhibited a typical polarization dependent loss as largeas 2 dB at 10-dB attenuation level. This is due to the largebirefringence of the antisymmetric cladding modes.

FIG. 23 shows one possible configuration that can reduce the inherentpolarization dependent loss of filter 10. In FIG. 23, double-pass filter100 consists of a 3-port circulator with input-, middle-, andoutput-port fibers, 12′, 12″ and 12′″, respectively, and Faradayrotating mirror (FRM) 104. The middle-port fiber 12″ is connected toacousto-optic tunable filter 10 and Faraday rotating mirror 104. Whenlight comes in through input-port fiber 12′ , it is directed to filter10, through circulator 102, and then refracted by Faraday rotatingmirror 104. Faraday rotating mirror 104 acts as a conjugate mirror withrespect to optical polarization states. So, if the light pass throughfilter 10 in a specific polarization state, then on the way back afterreflection it pass through filter 10 in its orthogonal polarizationstate. Since the light pass through filter 10 twice but in mutuallyorthogonal states, the total attenuation after the double pass becomespolarization-insensitive. Another benefit of the double passconfiguration is that, since the filtering takes place twice in filter10, the drive RF power applied to filter 10 to obtain a certainattenuation depth is reduced half compared to single-pass configurationas in FIG. 4. For instance, when filter 10 is operated at an attenuationdepth of 5 dB, the overall attenuation depth of double-pass filter 100becomes 10 dB.

Another embodiment of a device configuration for low polarizationdependence is shown in FIG. 24. In this embodiment, dual filter 110consists of filters 10′ and 10″ in tandem and connected through midfiber section 112. Filters 10′ and 10″ are preferably operated at thesame RF frequency. The birefringence of mid fiber section 112 isadjusted such that it acts as a half-wave plate aligned with 45-degreeangle with respect to the eigen polarization axes of filters 10′ and10″. In other words, the light passing through filter 10′ in one eigenpolarization state enters filter 10″ in the other eigen polarizationstate. If the polarization dependent loss is the same loss for filters10′ and 10″, the overall attenuation after passing through filters 10′and 10″ becomes polarization-insensitive. If filters 10′ and 10″ are notidentical in terms of polarization dependent loss, the double filter 110would exhibit residual polarization dependent loss that should be,however, smaller than the polarization dependent loss of individualfilters, 10′ or 10″. Therefore, it is desirable that filters 10′ and 10″are identical devices. Since the filtering takes place by two filters,the drive powers to individual filters are reduced, compared to using asingle filter alone, to achieve the same attenuation depth.

In one embodiment, illustrated in FIG. 23, circulator 102 based onmagneto-optic crystal has overall insertion loss and polarizationdependent loss of 1.5 dB and 0.5 dB, respectively. Faraday rotatingmirror 104 has insertion loss and polarization dependent loss of 0.5 dBand 0.5 dB, respectively. Curve 120 in FIG. 24(a) shows the polarizationdependent loss profile in one embodiment when filter 10 was operated toproduce 10-dB attenuation at 1550 nm. The filter profile in thisinstance is shown by curve 124 in FIG. 24(b). Optical fiber 12 used inthe filter was a conventional communication-grade single mode fiber.When the filter was used in the double-pass configuration, the overallpolarization dependent loss was reduced greatly as shown by curve 121 inFIG. 24(a).

The polarization dependent loss was reduced down to less than 0.2 dB.The total insertion loss of double-pass filter was 3 dB, mainly due tothe circulator and splices. In this embodiment, the drive power tofilter 10 required to produce total 10-dB attenuation at 1550 nm, asshown by filter profile 125 in FIG. 24(b), was decreased compared to thesingle-pass filter experiment.

In another embodiment, illustrated in FIG. 25, two filters werefabricated with a conventional circular-core single mode fiber. Eachfilter was operated with 5-dB attenuation at the same center wavelength,1550 nm. The overall dual filter profile is shown by curve 126 in FIG.24(b). In these filters, the eigen polarization states are linear andtheir axes are parallel and orthogonal to the direction of the flexuralacoustic wave vibration or the acoustic polarization axis. This isgenerally true with filters made of a circular-core fiber where thedominant birefringence axes are determined by the lobe orientation ofthe cladding mode, which is the same as the acoustic polarization axis.Linear axis orthogonal to acoustic polarization is the slow axis, andits orthogonal axis is the fast axis. In this embodiment, a polarizationcontroller was used in mid fiber section 112 and controlled to minimizethe overall polarization dependent loss of dual filter 110.

The loss profile is shown by curve 122 in FIG. 24(a). The total filterprofile is shown by curve 126 in FIG. 24(b). The residual polarizationdependent loss as large as 0.6 dB is primarily due to differentpolarization dependent loss of filters 10′ and 10″, and could be reducedgreatly if identical two filters were used.

Another important characteristic of filter 10 is the intensitymodulation of an optical signal passing through the filter. One reasonwhich gives rise to the intensity modulation of the output signal isstatic coupling between the core and cladding modes either bymicrobending of fiber 12 or imperfect splices, if present. Anotherreason is an acoustic wave propagating backward in interactive region 36by an acoustic reflection at imperfect acoustic damper 30 and fiberjacket 32. FIG. 26 shows an example of output signal 139 suffering fromthe intensity modulation by backward acoustic reflection at acousticdamper 30. In this case, the major modulation frequency is equal totwice the acoustic frequency. The modulation depth is defined by theratio of peak-to-peak AC voltage amplitude, V_(AC) to DC voltage,V_(DC). By static mode coupling, the major modulation frequency is equalto the acoustic frequency. When both static mode coupling and backwardacoustic wave are present, the intensity of the output is modulated atfrequencies of both first- and second-harmonics of the acousticfrequency. The modulation depth, when smaller than 20%, is,approximately, linearly proportional to the amount of attenuation in dBscale. In most WDM communication system applications, the modulationdepth is generally required to be less than 3% at 10-dB attenuationlevel.

In one embodiment, illustrated in FIG. 4, filter 10 was fabricated byusing a conventional single-mode fiber. The modulation depth at 10-dBattenuation level was about 10% at both first- and second-harmonics ofthe acoustic frequency, as shown by curves 140 and 141 in FIG. 27(a),respectively. The same filter was used as filter 10 in anotherembodiment, illustrated in FIG. 23. The RF drive power to the filter wascontrolled to produce 10-dB attenuation depth. In the first embodimentof double-pass filter 100, the length of fiber section 106 was selectedsuch that the round-trip travel time of fiber section 106 is equal to aquarter of the period of the acoustic wave. In this case, thesecond-harmonics component of the intensity modulation can becompensated out. Curves 142 and 143 in FIG. 27(a) show the modulationdepth of first- and second-harmonics components, respectively. Thesecond-harmonics was eliminated almost completely. The first-harmonicswas also reduced a little, which may be attributed to imperfect lengthmatching of fiber section 106. In the second embodiment of double-passfilter 100, the length of fiber section 106 was such that the opticalround-trip travel time of fiber section 106 is equal to a half of theperiod of the acoustic wave. In this case, the first-harmonics componentof the intensity modulation can be reduced. Curves 147 and 148 in FIG.27(b) show the modulation depth of first- and second-harmonicscomponents, respectively. The first-harmonics was eliminated almostcompletely.

Reduction of intensity modulation can also be achieved by dual filter110 where the length of mid fiber section 112 is selected properly. Forexample, if the first-harmonic modulation component is to becompensated, the length of mid fiber section 112 is such that theoptical travel time from one end of section 112 to the other end isequal to a half of the period of the acoustic wave.

Referring now to FIG. 28, one embodiment of the present invention is atunable VOA assembly 150 that includes a tunable VOA 152, coupled to atap coupler 154, a detector 156 and an attenuator control circuit 158that creates a local loop. A network loop is created by couplingattenuator control circuit 158 to network components in order to receivenetwork commands. Attenuator 152 can be a liquid crystal attenuator, aMEMS device, an acoustic-optic device, a Fabry Perot device, amechanical sliding attenuator, a magneto-optic device and the like.

Tap coupler 154 can be a fused directional coupler, a bulk optic filter,a grating positioned in a fiber, and the like. Detector 156 can be anyphotodetector well known to those skilled in the art.

In a preferred embodiment, VOA 152 couples light from a fundamental coremode of an optical fiber to a higher-order mode such as a higher ordercore mode or a cladding mode. The configuration of VOA 152 is preferablythe same as AOTF 10. The amount of coupling is determined by theamplitude of the acoustic wave. Transmission in the fundamental coremode is controlled by the voltage of an RF signal applied to thetransducer.

Optical tap coupler 154, optical power detector 156 , and an attenuatorcontrol circuit 158 provide a feedback loop to VOA 152. Additionally,the feedback signal can come from other system elements, not shown, thatare coupled with VOA system 150. Control circuit 158 can include adecision circuit and an RF generator. Control circuit 158 compares theoutput signal power determined from the detector output with a targetvalue required by a system operator. Control circuit 158 controls thevoltage of the RF signal that goes to the transducer of VOA 152 so thatthe optical signal power approaches the target value.

VOA assembly 150 is suitable with single or multiple wavelength channelsand/or bands, depending on the wavelength bandwidth of the AO couplingand the channel and/or band spacing. VOA assembly 150 provides broadbandoperation, spectral attenuation and broadband tilt adjustment. VOAassembly 150 can provide approximate flat spectral attenuation or it canprovide a tilt adjustment by moving the match to one side or the other.This can require network feedback from a spectral monitor. Further, twoVOA assemblies 150 can be in series, with one providing tile and theother overall attenuation.

Referring now to FIG. 29, a channel equalizer 159 is illustrated. Whenused for a single wavelength channel, VOA 152 is likely to be positionedin an optical node incorporating a demultiplexer 160, such as an arrayedwave-guide grating (AWG) router. For example, VOA 152 can be used toequalize the powers of multiple channels and/or bands including, inparticular, added channels and bands. In this case, the RF power andfrequency for each VOA 152 is set differently according to theattenuation desired for each channel and/or band VOA 152 is to dealwith. Polarization dependence of each VOA 152 is largely tolerated dueto the feedback operation as long as the feedback speed is faster thanthe polarization fluctuations. For example, the characteristic time ofSOP fluctuation in a real communication system can be on the order of amillisecond.

Demultiplexer 160 is configured to receive a plurality of different WDMchannels and separate the different signals (channels) into differentfibers, one fiber for each wavelength channel and/or bands. Eachseparate channel or band is individually attenuated with a VOA 152. Thisprovides individual control for each channel or band. Some of thechannels or bands can be dropped and not passed to VOA's 152. Newchannels or bands can be added after demultiplexer 160. VOA's 152provide gain flattening and also permit adding and dropping of channelsand/or bands. VOA's 152 provide spectral flattening and adjust thepowers so the recombined signals all have a predetermined power.

In the FIG. 29 embodiment, a first series of VOA's 152 is positionedbetween the drop and add and a second series of VOA's 152 positionedafter the add. A multiplexer 162 is positioned downstream from thesecond series of VOA's 152. A monitor 164 is coupled to the output.Monitor 164 can provide a feedback signal that is used to adjust VOA's152. The embodiment of FIG. 29 provides broadband operation, variablespectral attenuation, channel/band by channel/band spectral attenuationand broadband tilt adjustment.

Referring now to FIG. 30, one embodiment of an optical cross-connectapparatus 166 is provided. Optical cross-connect apparatus 166 provideschannel routing, switching and leveling between two inputs with multiplechannels or bands and two outputs of multiple channels or bands. Opticalcross-connect apparatus 166 includes demultiplexer 160, multiplexer 162,a demultiplexer 168 and a multiplexer 170 and coupled to optical crossconnect 172 which includes any number of devices to redirect channels orbands, including but not limited to mirrors and the like. Optical crossconnect 172 can be made using MEMS mirrors, bubble switch technology,liquid crystals and the like. A plurality of VOA's 152 are each coupledto an optical fiber and positioned between optical cross connect 172 andmultiplexers 162 and 170. VOA's 152 are included to individually adjustthe power of individual channels or bands and achieve leveling and/orspectral grooming.

Optionally included is a monitor 174 which can be a spectral monitor andthe like that monitors the spectral output. A system command device 176is coupled to a feedback control to receive network commands for afeedback loop. These network commands can, (i) come from the end of thelink after electrical detection and bit error rate measurements, (ii)come from spectral monitors located throughout the network and (iii) canbe IP signals or proprietary signals to the local controlelectronics/processor. A second monitor 178 is provided at the secondoutput. The embodiment illustrated in FIG. 30 also provides broadbandoperation, spectral attenuation and channel by channel broadbandspectral adjustment.

Referring now to FIG. 31, another embodiment of an optical cross-connectapparatus 180 includes two or more optical cross connects 172 and 182.At least two demultiplexers 160 and 184 are provided at the inputcarrying WDM signals. At least two multiplexers 162 and 184 are at theoutput. Each group goes to a cross connect. As illustrated in FIG. 31,there are two groups and each goes to optical cross connect 172 and 182.The channels or bands are split into two or more groups. Each group goesto a different cross connect.

There can be any number of different groups. Each channel or band can beits own group. In one embodiment, the number of channels or bands canequal the number of cross connects. For example all the λ1 go to onecross connect, λ2 to a second, λ3 to a third and the like. This preventstwo channels or bands from being on top of each other in one fiber. Allof the even channels or bands can be in one group and the odd channelsor bands in the other group.

In the FIG. 31 embodiment, one group of channels or bands is directed tooptical cross connect 172 and the other group is directed to opticalcross connect 182. Thereafter, multiplexers 162 and 184 combine thedifferent channels or bands which are directed to the different outputfibers. A plurality of VOA's 152 are coupled to optical cross connects172 and 182. Spectral monitors 174 couple the output fibers with VOA's152. Amplifiers 186 can be coupled to the optical fibers carrying theWDM signals. It will be appreciated that the embodiment of FIG. 31 canbe extended to any desired number of optical cross connects,demultiplexers and multiplexers.

For a wavelength channel or band it only one VOA is needed to beincluded between the demultiplexer and the multiplexer. Only one degreeof freedom is required for each channel or band. All the VOA's arepreferably at either the input or at the output of the cross connect.Alternatively, VOA's can be positioned at both the input and the output.Spectral monitors are preferably at the output fibers. The spectralmonitors send information to the feedback loop coupled to the VOA's.Network commands can go to the feedback control. This provides thetarget spectrum. Feedback control goes from a tap in the fiber, to thespectral monitor and then to the feedback control and then to the VOA.The feedback control has an input from the spectral monitor and anotherfrom the network commands which provides the target spectrum.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. An optical communication assembly, comprising: anoptical cross connect coupled to a first, a second, a third and a fourthset of optical fibers; a first demultiplexer coupled to a first inputfiber and the first set of optical fibers; a second demultiplexercoupled to a second input fiber and the second set of optical fibers; afirst multiplexer coupled to a first output fiber and the third set ofoptical fibers; a second multiplexer coupled to a second output fiberand the fourth set of optical fibers; and a first set of attenuatorscoupled to the third set of optical fibers and a second set ofattenuators coupled to the fourth set of optical fibers.
 2. The assemblyof claim 1, wherein the optical cross connect provides channel or bandrouting between the first and second input fibers to the first andsecond output fibers.
 3. The assembly of claim 1, wherein the opticalcross connect provides channel or band routing and switching between thefirst and second input fibers to the first and second output fibers. 4.The assembly of claim 1, wherein the optical cross connect provideschannel or band routing, switching and leveling between the first andsecond input fibers to the first and second output fibers.
 5. Theassembly of claim 1, further comprising: a first spectral monitorcoupled to the first output fiber and at least a portion of the firstset of attenuators.
 6. The assembly of claim 5, further comprising: afirst network command device coupled to the first spectral monitor andthe at least a portion of the first set of attenuators, the firstspectral monitor and the first network command device forming a firstclosed feedback loop.
 7. The assembly of claim 6, further comprising: asecond spectral monitor coupled to the second output fiber and at leasta portion of the second set of attenuators.
 8. The assembly of claim 7,further comprising: a second network command device coupled to thesecond spectral monitor and the at least a portion of the second set ofattenuators, the second spectral monitor and the second network commanddevice forming a second closed feedback loop.
 9. The assembly of claim1, wherein the first and second demultiplexers are each an arrayedwave-guide grating.
 10. The assembly of claim 1, wherein each of thefirst and second demultiplexers is configured to receive a plurality ofdifferent WDM channels or bands and direct the different channels orbands into the first and second sets of optical fibers.
 11. The assemblyof claim 1, wherein each optical fiber of the first and second sets ofoptical fibers receives a different channel or band from the first andsecond demultiplexers.
 12. The assembly of claim 1, wherein eachattenuator of the first and second sets of attenuators is a variableoptical attenuator.
 13. The assembly of claim 1, wherein each attenuatoris coupled to a single channel or band.
 14. The assembly of claim 1,wherein the power of each channel or band passed by the firstmultiplexer and the first output fiber, and the second multiplexer andthe second output fiber is individually controllable.
 15. The assemblyof claim 1, wherein each acoustic wave generator produces multipleacoustic signals with individual controllable strengths and frequenciesand each of the acoustic signals provides a coupling between the coremode and a different spatial mode.
 16. The assembly of claim 1, whereineach acoustic wave generator generates a transverse wave.
 17. Theassembly of claim 1, wherein each acoustic wave generator generates alongitudinal wave.
 18. The assembly of claim 1, wherein each acousticwave generator generates a torsional wave.
 19. The assembly of claim 1,wherein the attenuator optical fiber has a single core mode guided alongthe core.
 20. The assembly of claim 1, wherein the attenuator opticalfiber has a multiple modes guided along the core.
 21. The assembly ofclaim 1, wherein the attenuator optical fiber provides fundamental andcladding mode propagation.
 22. The assembly of claim 1, wherein theattenuator optical fiber is sufficiently tensioned to reduce losses inthe acoustic wave.
 23. The assembly of claim 1, further comprising: aspectral monitor positioned at an output of the multiplexer.
 24. Theassembly of claim 1, further comprising: a plurality of detectorscoupled to the plurality of attenuators.
 25. The assembly of claim 1,further comprising: an acoustic damper coupled to each attenuatoroptical fiber.
 26. An optical communication assembly, comprising: anoptical cross connect coupled to a first, a second, a third and a fourthset of optical fibers; a first demultiplexer coupled to a first inputfiber and the first set of optical fibers; a second demultiplexercoupled to a second input fiber and the second set of optical fibers; afirst multiplexer coupled to a first output fiber and the third set ofoptical fibers; a second multiplexer coupled to a second output fiberand the fourth set of optical fibers; and a first set of attenuatorscoupled to the first set of optical fibers and a second set ofattenuators coupled to the second set of optical fibers.
 27. Theassembly of claim 26, wherein the optical cross connect provides channelor band routing between the first and second input fibers to the firstand second output fibers.
 28. The assembly of claim 26, wherein theoptical cross connect provides channel or band routing and switchingbetween the first and second input fibers to the first and second outputfibers.
 29. The assembly of claim 26, wherein the optical cross connectprovides channel or band routing, switching and leveling between thefirst and second input fibers to the first and second output fibers. 30.The assembly of claim 26, further comprising: a first spectral monitorcoupled to the first output fiber and at least a portion of the firstset of attenuators.
 31. The assembly of claim 30, further comprising: afirst network command device coupled to the first spectral monitor andthe at least a portion of the first set of attenuators, the firstspectral monitor and the first network command device forming a firstclosed feedback loop.
 32. The assembly of claim 31, further comprising:a second spectral monitor coupled to the second output fiber and atleast a portion of the second set of attenuators.
 33. The assembly ofclaim 32, further comprising: a second network command device coupled tothe second spectral monitor and the at least a portion of the second setof attenuators, the second spectral monitor and the second networkcommand device forming a second closed feedback loop.
 34. The assemblyof claim 26, wherein the first and second demultiplexers are each anarrayed wave-guide grating.
 35. The assembly of claim 26, wherein eachof the first and second demultiplexers is configured to receive aplurality of different WDM channels or bands and direct the differentchannels or bands into the first and second sets of optical fibers. 36.The assembly of claim 26, wherein each optical fiber of the first andsecond sets of optical fibers receives a different channel or band fromthe first and second demultiplexers.
 37. The assembly of claim 26,wherein each attenuator of the first and second sets of attenuators is avariable optical attenuator.
 38. The assembly of claim 26, wherein eachattenuator is coupled to a single channel or band.
 39. The assembly ofclaim 26, wherein the power of each channel or band passed by the firstmultiplexer and the first output fiber, and the second multiplexer andthe second output fiber is individually controllable.
 40. The assemblyof claim 26, wherein each acoustic wave generator produces multipleacoustic signals with individual controllable strengths and frequenciesand each of the acoustic signals provides a coupling between the coremode and a different spatial mode.
 41. The assembly of claim 26, whereineach acoustic wave generator generates a transverse wave.
 42. Theassembly of claim 26, wherein each acoustic wave generator generates alongitudinal wave.
 43. The assembly of claim 26, wherein each acousticwave generator generates a torsional wave.
 44. The assembly of claim 26,wherein the attenuator optical fiber has a single core mode guided alongthe core.
 45. The assembly of claim 26, wherein the attenuator opticalfiber has a multiple modes guided along the core.
 46. The assembly ofclaim 26, wherein the attenuator optical fiber provides fundamental andcladding mode propagation.
 47. The assembly of claim 26, wherein theattenuator optical fiber is sufficiently tensioned to reduce losses inthe acoustic wave.
 48. The assembly of claim 26, further comprising: aspectral monitor positioned at an output of the multiplexer.
 49. Theassembly of claim 26, further comprising: a plurality of detectorscoupled to the plurality of attenuators.
 50. The assembly of claim 26,further comprising: an acoustic damper coupled to each attenuatoroptical fiber.
 51. An optical communication assembly, comprising: ademultiplexer coupled to an input fiber; a multiplexer coupled to anoutput fiber; a plurality of optical fibers, each of an optical fiberbeing coupled to one or both of the demultiplexer and multiplexer; and aplurality of attenuators each coupled to an optical fiber of theplurality of optical fibers, each attenuator including, an attenuatoroptical fiber with a longitudinal axis, a core and a cladding in asurrounding relationship to the core, the attenuator optical fiberhaving a plurality of guided core modes, an acoustic wave propagationmember with a proximal end and a distal end, the distal end beingcoupled to the attenuator optical fiber, the acoustic wave propagationmember propagating an acoustic wave from the proximal to the distal endand launch an acoustic wave in the attenuator optical fiber, at leastone acoustic wave generator coupled to the proximal end of the acousticwave propagation member.
 52. The assembly of claim 51, wherein thedemultiplexer is an arrayed wave-guide grating.
 53. The assembly ofclaim 51, wherein the demultiplexer is configured to receive a pluralityof different WDM channels or bands and direct the different channels orbands into optical fibers of the plurality of optical fibers.
 54. Theassembly of claim 53, wherein each optical fiber receives a differentchannel or band from the demultiplexer.
 55. The assembly of claim 51,wherein each attenuator is a variable optical attenuator.
 56. Theassembly of claim 51, wherein the optical communication assembly is achannel equalizer.
 57. The assembly of claim 51, wherein at least aportion of the channels or bands passed by the multiplexer and theoutput fiber are individually attenuated by one of the attenuators inthe plurality of attenuators.
 58. The assembly of claim 51, wherein atleast a portion of the channels or bands passed by the multiplexer andthe output fiber are individually controllable.
 59. The assembly ofclaim 51, wherein at least one optical fiber of the plurality of opticalfibers is not coupled to the demultiplexer and adds at least one channelor band.
 60. The assembly of claim 51, wherein at least one opticalfiber of the plurality of optical fibers is not coupled to themultiplexer and drops at least one channel or band introduced to thedemultiplexer by the input fiber.
 61. The assembly of claim 51, whereineach acoustic wave generator produces multiple acoustic signals withindividual controllable strengths and frequencies and each of theacoustic signals provides a coupling between the core mode and adifferent spatial mode.
 62. The assembly of claim 51, wherein a lengthof the attenuator optical fiber is no greater than 1 meter.
 63. Theassembly of claim 51, wherein each acoustic wave generator generates atransverse wave.
 64. The assembly of claim 51, wherein each acousticwave generator generates a longitudinal wave.
 65. The assembly of claim51, wherein each acoustic wave generator generates a torsional wave. 66.The assembly of claim 51, wherein the attenuator optical fiber has asingle core mode guided along the core.
 67. The assembly of claim 51,wherein the attenuator optical fiber has multiple core modes guidedalong the core.
 68. The assembly of claim 51, wherein the attenuatoroptical fiber provides fundamental and cladding mode propagation. 69.The assembly of claim 51, further comprising: a spectral monitorpositioned at an output of the multiplexer.
 70. The assembly of claim51, further comprising: a plurality of detectors coupled to theplurality of attenuators.
 71. The assembly of claim 51, furthercomprising: an acoustic damper coupled to each attenuator optical fiber.